A method for manufacturing a stiffened panel made from composite material comprising a skin and elongated reinforcing elements wherein some of the elongated reinforcing elements cross each other. The method comprises the steps of (a) laying up a flat laminate comprising stacked plies of composite layers for forming a structure comprising the elongated reinforcing elements of the panel, (b) cutting the flat laminate along intersection lines of planes defining the webs of two crossing reinforcing elements, (c) cutting in the flat laminate the outline of the elongated reinforcing elements, (d) forming the structure comprising the elongated reinforcing elements of the panel, (e) laying up plies of composite layers for forming the skin, and (f) curing the laid-up plies of the skin and the formed structure comprising the elongated reinforcing elements.